The present invention relates generally to wireless telecommunications, and specifically to advanced cellular telephone networks.
The Global System for Mobile (GSM) telecommunications is used in cellular telephone networks in many countries around the world. GSM offers a useful range of network services and standards. Existing GSM networks are based on time-division multiple access (TDMA) digital communications technology. In a TDMA-based cellular network, each mobile subscriber unit communicates with only a single base station at any given time. When a subscriber moves from one cell to another, a xe2x80x9chard handoverxe2x80x9d takes place, in which the base station with which the subscriber has been communicating breaks off its link with the subscriber, and a new base station takes over.
Code-division multiple access (CDMA) is an improved digital communications technology, which affords more efficient use of radio bandwidth than TDMA, as well as a more reliable, fade-free link between cellular telephone subscribers and base stations. The leading CDMA standard is IS-95, promulgated by the Telecommunications Industry Association (TIA). This standard provides xe2x80x9csoft handoverxe2x80x9d (or xe2x80x9chandoffxe2x80x9d) capability, wherein in moving from one cell to another, the subscriber unit is temporarily in contact with two or more base stations at the same time. This soft handover, which is made possible by the code-division approach, decreases the likelihood of a loss of connection, which can happen frequently in hard handovers.
PCT patent application PCT/US96/20764, which is incorporated herein by reference, describes a wireless telecommunications system that uses a CDMA air interface (i.e., basic RF communications protocols) to implement GSM network services and protocols. Using this system, at least some of the TDMA base stations (BSSs) and subscriber units of an existing GSM network would be replaced or supplemented by corresponding CDMA equipment. CDMA BSSs in this system are adapted to communicate with GSM mobile switching centers (MSCs) via a standard GSM A-interface. The core of GSM network services is thus maintained, and the changeover from TDMA to CDMA is transparent to users.
Hybrid cellular communications networks, incorporating both GSM and CDMA elements, are also described in PCT patent publications WO 95/24771 and WO 96/21999, and in an article by Tscha, et al., entitled xe2x80x9cA Subscriber Signaling Gateway between CDMA Mobile Station and GSM Mobile Switching Center,xe2x80x9d in Proceedings of the 2nd International Conference on Universal Personal Communications, Ottawa (1993), pp. 181-185, which are incorporated herein by reference. None of these publications deals with specific issues of how to perform efficient handovers of subscriber units between different base stations in such hybrid networks.
PCT patent application PCT/US97/00926, which is also incorporated herein by reference, describes methods of intersystem handover between CDMA and TDMA BSSs in a hybrid GSM/CDMA telecommunications system. A GSM/TDMA BSS generates pilot beacon signals in accordance with CDMA technology. During a telephone call, a subscriber unit detects the pilot signals and notifies a base station controller that the signals have been detected. The subscriber unit is then handed over from the CDMA to the TDMA BSS without interrupting the call.
It is an object of the present invention to provide methods and apparatus for use in a mixed TDMA/CDMA cellular communications network.
It is a further object of some aspects of the present invention to provide improved methods and apparatus enabling handover of a subscriber unit between TDMA and CDMA base stations without interrupting communications.
In preferred embodiments of the present invention, a mixed GSM/CDMA cellular communications system includes both TDMA and CDMA base stations, jointly controlled by a mobile switching center (MSC). Systems of this type are described generally in the above-mentioned PCT patent applications, which are incorporated herein by reference. A subscriber unit in the system, also referred to herein as a mobile station (MS), is capable of communicating with both types of base stations, by appropriately switching between TDMA and CDMA air interfaces, while preferably using GSM network protocols over both types of interface. It is a feature of preferred embodiments of the present invention that the communications system may be based on an existing GSM/TDMA infrastructure, with the addition of CDMA BSSs, and with substantially no other modification to the existing infrastructure.
In order to determine when a handover should take place, a MS in communication with a current base station of one type (CDMA or TDMA) monitors RF signals originating from another base station, which may be a base station of the other type (TDMA or CDMA, respectively). A message sequence between the current base station and the MS enables the MS to acquire appropriate synchronization information with regard to the new base station, and report back on this information to the current base station. The information is used by the system to enables the MS to establish an air interface with the new base station, whereupon the handover takes place without substantially interrupting communications between the MS and the network.
In the context of the present patent application, such handovers between base stations are referred to as xe2x80x9cmobile-assisted handovers.xe2x80x9d Mobile-assisted handover is used in GSM and in CDMA systems known in the art, wherein a mobile station measures and reports on the strength of signals received from a base station transceiver in a neighboring cell before being handed over to that cell. In hybrid GSM/CDMA systems that have been proposed to date, however, mobile stations are presumed to be capable of receiving signals from either a CDMA or a TDMA base station at any given time (or a CDMA beacon associated with a TDMA base station, as in the above-mentioned PCT patent application PCT/US97/00926), but not both, and are therefore not capable of providing this type of assistance. The provision of mobile-assistance in accordance with the principles of the present invention enables handovers to be conducted more smoothly and reliably than would otherwise be possible.
In some preferred embodiments of the present invention, the MS switches between TDMA and CDMA operation in the course of a telephone call, according to instructions received from the base station with which the unit is in communication. Before the handover is to take place, the MS receives signals from both TDMA and CDMA base stations, and reports back to the base station regarding the signals it is receiving. The information thus reported is reported back to and used by the BSC to initiate the handover. Preferably, the MS comprises a single radio transceiver, and therefore, at any given moment the MS can communicate with either the TDMA or CDMA base station, but not both. (In accordance with the principles of IS-95, however, as described hereinabove, the unit can communicate with more than one CDMA base station at once.) It is noted further that each GSM/TDMA base station has its own synchronization clock, to which the MSs in communication therewith are synchronized, while the CDMA base stations are mutually synchronized to a real time of day. Therefore, in switching between the TDMA and CDMA stations, the MS in each case acquires and synchronizes its operation to the appropriate clock signal without substantially interrupting the telephone call.
In some of these preferred embodiments, the MS is in communication with a CDMA base station, when it is determined that the unit may be handed over to a GSM/TDMA base station. CDMA transmission by the MS transceiver is interrupted temporarily, during which time the unit performs a GSM neighbor scan, generally in accordance with GSM standards, to acquire and synchronize to the TDMA base station. Preferably, the CDMA transmission is interrupted for a single frame, typically 20 ms long, creating an idle time slot in accordance with the IS95 standard. After the TDMA base station is identified, and suitable messages have been exchanged, a traffic channel between the base station is opened, and the MS is switched to the TDMA base station while interruption of a telephone call being conducted by the MS is substantially minimized.
In others of these preferred embodiments, the MS is in communication with a TDMA base station, when it is determined that the unit may be handed over to a CDMA base station. In order to synchronize with the CDMA station, the MS acquires the time of day, preferably by receiving an accurate time of day from the TDMA base station, wherein the GSM network is provided with equipment necessary to generate and broadcast the time of day. Preferably, the network includes a cell broadcast system (CBS), in accordance with the GSM standard, which is used to receive the time of day, provided, for example, by the Global Positioning System (GPS) or received from one or more of the CDMA base stations, and broadcast it through the network to the MSs. Alternatively, the MS temporarily interrupts TDMA reception in order to acquire and synchronize to the time of day of the CDMA station. Thus, although a certain degradation of the signal may result from the TDMA time slot(s) lost in this fashion, the mobile-assisted handover from TDMA to CDMA is generally more reliable and less disturbing to a user of the MS than would otherwise be possible.
Although preferred embodiments are described herein with reference to MSs having a single transceiver for TDMA and CDMA use, it will be appreciated that the principles of the present invention may similarly be applied using subscriber units and system hardware of other types, and particularly using a subscriber unit having separate or only partially integrated TDMA and CDMA transceivers.
There is therefore provided, in accordance with a preferred embodiment of the present invention, in a mobile wireless telecommunications system, which includes base stations of a first type operating according to a first air interface, and base stations of a second type operating according to a second air interface, a method for handing over a mobile station in the system from a first base station, which is of the first type, to a second base station, which is of the second type, including:
establishing a communications link over the first air interface between the mobile station and the first base station;
receiving data from the mobile station responsive to a signal received by the mobile station over the second air interface from the second base station, substantially without breaking the communications link with the first base station; and
handing over the mobile station from the first to the second base station responsive to the data received therefrom.
Preferably, receiving the data includes receiving a measurement of signal strength, and handing over the mobile station includes comparing measurements of signal strengths from the first and second base stations and handing over the mobile station responsive to the comparison. Preferably, receiving the data includes applying a weighting factor to the measurement of signal strength, wherein applying the weighting factor includes varying the factor according to a network condition in the system. Further preferably, applying the weighting factor includes transmitting a weighting factor over the communications link to the mobile station, which applies the weighting factor to the measurement.
Preferably, receiving the data includes receiving an identification of the second base station based on decoding by the mobile station of the signal received over the second air interface.
In a preferred embodiment transmitting from the first base station to the mobile station a list of frequencies of base stations of the second type in the system, such that the mobile station seeks to receive the signal at a frequency in the list.
Preferably, handing over the mobile station includes transmitting a handover command from the first base station. In a preferred embodiment handing over the mobile station includes sending an initial transmission over the second air interface responsive to the handover command, and the method includes reacquiring the communications link over the first air interface if the initial transmission over the second air interface is not successfully received.
Preferably, transmitting the handover command includes transmitting a command over the first air interface that encapsulates parameters relating to the second air interface. Most preferably, transmitting the command includes transmitting a command in accordance with a GSM standard that encapsulates parameters defined in accordance with an IS-95 standard, wherein the parameters encapsulated include an IS-95 long code.
Preferably, establishing the communications link and receiving the data responsive to the signal include establishing the link and receiving the signal at the mobile station using a single RF transceiver in the mobile station.
In a preferred embodiment, one of the first and second air interfaces includes a TDMA interface, and the other of the interfaces includes a CDMA interface, wherein the TDMA interface preferably includes a GSM interface, and wherein the CDMA interface is configured to convey GSM network messages. Preferably, the CDMA interface is based on an IS-95 standard.
Preferably, establishing the communications link includes using a single radio resource management protocol layer to manage the first air interface, and wherein handing over the mobile station includes using the single radio resource management protocol layer to manage the second air interface.
Further preferably, receiving the data from the mobile station includes defining an area of overlap between a first region served by the first air interface and a second region served by the second air interface, and triggering the mobile station to receive the data when the mobile station is in the area of overlap.
In a preferred embodiment, the first air interface includes a CDMA interface, and wherein the second air interface includes a GSM/TDMA interface, and receiving data from the mobile station includes gating the mobile station to interrupt a CDMA communications link so as to receive and decode a GSM/TDMA signal. Preferably, gating the mobile station includes interrupting CDMA communications for the duration of an IS-95 frame, wherein receiving the data includes receiving an identification of the second base station based on decoding of GSM frequency correction and synchronization channels of the signal by the mobile station.
In another preferred embodiment the first air interface includes a GSM/TDMA interface, and the second air interface includes a CDMA interface, and receiving the data from the mobile station includes controlling the mobile station to interrupt the communications link so as to receive and decode a CDMA signal.
Preferably, receiving the data includes conveying time of day information through the GSM/TDMA interface. Further preferably, conveying the time of day information includes broadcasting time of day information through the system using a GSM cell broadcast service, wherein broadcasting the time of day information includes receiving a time of day and an associated GSM frame number from a transceiver in communication with a base station of the first type in the system. Preferably, the mobile station decodes a sync channel of the CDMA signal so as to derive the time or day.
Alternatively or additionally, receiving the data includes conveying a GSM cell broadcast service message to the mobile station to initiate a search by the mobile station for a signal from a base station of the second type. Preferably, conveying the GSM cell broadcast service message to the mobile station includes conveying the message so as to be received by the mobile station while the mobile station is operating in a dedicated mode.
Preferably, receiving the data from the mobile station includes receiving an identification of a CDMA pilot beam decoded by the mobile station. Further preferably, the method includes mapping the second base station as a GSM base station so as to control the handover.
Preferably, controlling the mobile station includes controlling the mobile station to receive the CDMA signal during a first TDMA time slot and to decode the signal during a subsequent TDMA time slot while communicating with the base station over the TDMA interface so as to generate the data to be received by the base station.
There is further provided, in accordance with a preferred embodiment of the present invention, a method for conveying time of day information to a mobile station in a GSM wireless telecommunications system, including:
inputting the time of day information to the system; and
broadcasting the information to the mobile station over the system.
Preferably, the GSM wireless telecommunications system includes a cell broadcast system, and broadcasting the time of day information includes broadcasting the information over the cell broadcast system. Preferably, broadcasting the time of day information includes broadcasting a message so as to be received by the mobile station while the station is operating in a dedicated mode.
Further preferably, broadcasting the time of day information includes receiving a time of day and an associated GSM frame number from a transceiver in communication with the system, and the method includes synchronizing the mobile station to a CDMA transmission signal using the time of day information.
In a preferred embodiment the method includes determining a location of the mobile station responsive to a transmission thereby of the time of day information to a plurality of base stations in the system.
Preferably, inputting the time of day includes opening a data call from a transceiver having the time of day information to the cell broadcast center, wherein opening the data call preferably includes receiving time of day information from a GPS device. Alternatively, opening the data call includes receiving time of day information from a CDMA cell associated with the GSM system.
There is further provided, in accordance with a preferred embodiment of the present invention, in a GSM mobile wireless telecommunications system, which includes a first base station subsystem and a second base station subsystem, at least one of which subsystems operates according to a CDMA air interface, a method for handing over a mobile station in the system from first to the second base station subsystem, including:
mapping the at least one of the first and second subsystems that operates according to the CDMA air interface as a GSM/TDMA subsystem;
establishing a communications link between the mobile station and the first base station subsystem, so that the mobile station receives a first signal from the first base station subsystem;
receiving data from the mobile station responsive to a second signal received by the mobile station from the second base station subsystem, substantially without breaking the communications link with the first base station subsystem;
comparing the strengths of the first and second signals, substantially as though both the first and second base station subsystems were GSM/TDMA subsystems; and
handing over the mobile station from the first to the second base station subsystem responsive to comparison of the signal strengths.
Preferably, mapping the at least one of the subsystems that operates according to the CDMA air interface includes assigning to the subsystem a GSM frequency and location.
Further preferably, establishing the communications link and handing over the mobile station include conveying messages between the first and second subsystems and a mobile switching center in the system via a GSM A-interface. Preferably, both the first and second base station subsystems operate according to the CDMA air interface, wherein handing over the mobile station includes conveying a new IS-95 long code through the A-interface, substantially without violating A-interface protocols.
Preferably, receiving the data from the mobile station includes applying a weighting factor to the second signal, and wherein comparing the strengths of the signals includes comparing the weighted signal, wherein applying the weighting factor includes conveying the weighting factor to the mobile station, which applies the weighting factor to the second signal. Preferably, applying the weighting factor includes varying the factor according to a network condition in the system.
There is also provided, in accordance with a preferred embodiment of the present invention, wireless communications apparatus, for use in a mobile telecommunications system, including:
a base station of a first type which transmits and receives a first signal according to a first air interface;
a base station of a second type which transmits and receives a second signal according to a second air interface; and
a mobile station, which receives the second signal over the second air interface from the base station of the second type while maintaining a communication link over the first air interface with the base station of the first type, and which transmits data to the base station of the first type responsive to the second signal so that the mobile station is handed over from the first to the second base station responsive to the transmitted data.
Preferably, the data transmitted by the mobile station includes a measurement of signal strength, such that the mobile station is handed over responsive to a comparison of signal strengths of the first and second signals. Preferably, a weighting factor is applied to the measurement of signal strength, wherein the weighting factor is varied according to a network condition in the system. Preferably, the weighting factor is transmitted over the communications link to the mobile station, which applies the weighting factor to the measurement.
Further preferably, the mobile station decodes the second signal to determine an identification of the base station of the second type.
Preferably, the base station of the first type transmits to the mobile station a list of frequencies of mobile stations of the second type in the system, such that the mobile station seeks to receive the second signal at a frequency in the list.
Preferably, the base station of the first type transmits a handover command to the mobile station, whereby the mobile station is handed over from the first to the second base station. In a preferred embodiment an initial transmission is sent over the second air interface responsive to the handover command, and the mobile station reacquires the communications link over the first air interface if the initial transmission over the second air interface is not successfully received.
Preferably, the handover command encapsulates parameters relating to the second air interface. Most preferably, the command in substantially in accordance with a GSM standard and encapsulates parameters defined in accordance with an IS-95 standard, wherein the parameters encapsulated include an IS-95 long code.
Further preferably, the mobile station includes a single RF transceiver which communicates with both the base stations of the first and second types.
In a preferred embodiment, one of the first and second air interfaces includes a TDMA interface, and the other of the interfaces includes a CDMA interface., wherein the TDMA interface preferably includes a GSM interface, and wherein the CDMA interface is configured to convey GSM network messages. Preferably, the CDMA interface is based on an IS-95 standard. Further preferably, the mobile station uses a single radio resource management protocol layer to manage both the first and second air interfaces.
Preferably, the base station triggers the mobile station to receive the second signal over the second air interface when the mobile station is in an area of overlap between a first region served by the first air interface and a second region served by the second air interface
In a preferred embodiment, the first air interface includes a CDMA interface, and the second air interface includes a GSM/TDMA interface, and the base station of the first type gates the mobile station to interrupt the communications link so as to receive and decode a GSM signal.
Preferably, the mobile station interrupts the link for the duration of an IS-95 frame.
Further preferably, the mobile station processes the second signal to decode GSM frequency correction and synchronization channels of the signal.
In another preferred embodiment the first air interface includes a GSM/TDMA interface, and the second air interface includes a CDMA interface, and the base station of the first type controls the mobile station to interrupt the communications link so as to receive and decode a CDMA signal.
Preferably, the base station of the first type conveys time of day information to the mobile station through the GSM/TDMA interface. Preferably, the apparatus includes a GSM cell broadcast center, which conveys the time of day information through the system to the mobile station using a GSM cell broadcast service, wherein the cell broadcast center receives the time of day information and an associated GSM frame number from a transceiver in communication with a base station of the first type in the system.
Alternative or additionally, the mobile station decodes a synchronization channel of the CDMA signal so as to derive the time of day.
Preferably, the GSM cell broadcast center conveys a cell broadcast service message to the mobile station to initiate a search by the mobile station for the second signal, wherein the mobile station receives the cell broadcast service message while the mobile station is operating in a dedicated mode.
Alternatively or additionally, the mobile station processes the CDMA signal to identify a CDMA pilot beam.
Preferably, the mobile station receives the CDMA signal during a first TDMA time slot and processes the signal during a subsequent TDMA time slot while communicating with the base station over the TDMA interface so as to generate the data for transmission to the base station.
There is further provided, in accordance with a preferred embodiment of the present invention, apparatus for conveying time of day information to a mobile station in a GSM wireless telecommunications system, including a cell broadcast center, which broadcasts the information to the mobile station using a GSM cell broadcast system.
Preferably, the apparatus includes a transceiver in communication with the system, which transmits a time of day and an associated GSM frame number to the cell broadcast center, wherein the transceiver opens a data call through the system to the cell broadcast center so as to convey the time of day and the associated frame number thereto.
Preferably, the mobile station is synchronized to a CDMA transmission signal using the time of day information.
Further preferably, the mobile station receives the information from the cell broadcast system while operating in a dedicated mode.
There is moreover provided, in accordance with a preferred embodiment of the present invention, apparatus for inputting time of day information to a communications controller in a wireless telecommunications system, including:
a clock signal receiver, which receives the time of day information from a clock source; and
a radio transceiver, which receives the time of day information from the clock signal receiver, and which opens a data call through the system to the communications controller so as to convey the information thereto.
Preferably, the communications controller includes a GSM cell broadcast center, wherein the radio transceiver receives a GSM frame number from a base station in the system, and conveys the frame number to the cell broadcast center together with the time of day information.
Preferably, the clock signal receiver includes a radio receiver which receives the time of day information from a CDMA communications cell, wherein the radio transceiver includes the radio receiver.
Alternatively, the clock signal receiver includes a GPS device.
There is additionally provided, in accordance with a preferred embodiment of the present invention, apparatus for mobile wireless telecommunications in a GSM telecommunications system, including:
a mobile station; and
first and second base station subsystems, transmitting first and second signals to the mobile station, at least one of which is a CDMA signal, and both of which subsystems are mapped in the GSM system as GSM base station subsystems,
wherein the mobile station is handed over from the first to the second subsystem responsive to a comparison of the strengths of the first and second signals received by the mobile station, substantially as though both the first and second base station subsystems operated according to a GSM/TDMA air interface.
Preferably, the subsystem transmitting the CDMA signal is assigned a GSM frequency and location in the system. Further preferably, messages are conveyed between the first and second subsystems and a mobile switching center in the system via a GSM A-interface, wherein both the first and second signals include CDMA signals. Preferably, a new IS-95 long code is conveyed through the A-interface from the second to the first subsystem in order to hand over the mobile station, substantially without violating A-interface protocols.
Preferably, the mobile station applies a weighting factor to the second signal before the signal strengths are compared.
There is further provided, in accordance with a preferred embodiment of the present invention, a mobile station for use in a wireless telecommunications system including CDMA and TDMA base stations, including:
a single mobile radio transceiver, which communicates with the CDMA and TDMA base stations; and
a modem unit which encodes signals for transmission by the mobile transceiver and decodes signal received thereby, such that the signals are CDMA-encoded for communication with the CDMA base station and TDMA-encoded for communication with the TDMA base station.
Preferably, the modem unit encodes the signals in accordance with GSM radio interface layer protocols.
Further preferably, the mobile station receives and processes a signal from one of the CDMA and TDMA base stations substantially without breaking a communications link existing between the mobile station and the other one of the CDMA and TDMA base stations.
There is also provided, in accordance with a preferred embodiment of the present invention, a method for conveying messages to a plurality of mobile stations operating in a dedicated mode in a GSM wireless telecommunications system including a cell broadcast service, including:
broadcasting the messages to the mobile stations over the cell broadcast service; and
receiving the messages at the mobile stations substantially without terminating the dedicated mode operation of the mobile stations.
Preferably, broadcasting the messages includes sending time-of-day information or, alternatively or additionally, broadcasting a search trigger message.
There is additionally provided, in accordance with a preferred embodiment of the present invention, apparatus for mobile wireless telecommunications in a GSM telecommunications system, including:
a cell broadcast center, which broadcasts messages over a cell broadcast system; and
a mobile station, which receives the messages while communicating in a dedicated mode, substantially without terminating the dedicated mode communications.
Preferably, the cell broadcast center broadcasts time-of-day information or, alternatively or additionally, a search trigger message.
There is additionally provided, in accordance with a preferred embodiment of the present invention, a mobile station for use in a wireless telecommunications system including CDMA and TDMA base stations, including:
at least one mobile radio transceiver, which communicates with the CDMA and TDMA base stations; and
a modem unit which processes signals for transmission by the at least one transceiver and received thereby in accordance with a communications protocol stack, such that the signals are CDMA-encoded for communication with the CDMA base station and TDMA-encoded for communication with the TDMA base station, the stack including a single radio resource management protocol layer which controls the communications with both the CDMA and TDMA base stations.
Preferably, the radio resource management protocol layer performs substantially all of the functions of a GSM Radio Interface Layer 3 RR sublayer.
Further preferably, the radio resource management protocol layer controls a handover of the mobile station from one of the base stations to another of the base stations.
There is moreover provided, in accordance with a preferred embodiment of the present invention, in a GSM mobile wireless telecommunications system, which includes base station subsystems at least some of which operate according to a CDMA air interface, a method for controlling communications of a mobile station in the system with the base station subsystems, including:
sending and receiving signals between the mobile station and one of the base station subsystems over the CDMA air interface; and
controlling the sending and receiving using a radio resource management communications protocol layer that performs substantially all of the functions of a GSM Radio interface Layer 3 RR sublayer.
Preferably, the system further includes base station subsystems which operate according to a TDMA air interface and the method includes:
sending and receiving signals between the mobile station and one of the base station subsystems over the TDMA air interface,
wherein controlling the sending and receiving includes using the single radio resource management communications protocol layer to control sending and receiving of signals over both the CDMA and TDMA air interfaces.
Further preferably, the method includes handing over the mobile station between TDMA and CDMA base stations, wherein the handover is controlled by the radio resource management communications protocol layer.
The present invention will be more fully understood from the following detailed description of the preferred embodiments thereof, taken together with the drawings in which: